1. Field of the Invention
The invention relates to a method of transmitting, to a central recording system, seismic data collected by data acquisition devices spread out in a zone to be prospected, and the device for implementing same.
Modern seismic prospection methods include the use of data devices spaced apart at regular intervals over a distance sometimes of several kilometers. They are each adapted for collecting seismic signals picked up by one or more appropriate receivers (hydrophones or geophones) in response to the vibrations transmitted into the ground by a seismic source and reflected back by the discontinuities of the subsoil. The signals which they collect are sampled, digitized and stored in a memory before transmission thereof in real or delayed time to a central control and recording laboratory.
The different acquisition devices may be connected to the central control and recording laboratory by common cables adapted for transmitting both control and test signals and the accumulated seismic data. The different acquisition devices are interrogated in sequence by the central laboratory and in response transmit thereto the stored digital data.
2. Description of the Prior Art
Such a system is described for example in the U.S. Pat. No. 4 398 271.
The different acquisition devices may be connected to the central laboratory by a short wave link. Each of them is then associated with radio equipment. The collected data may be transmitted to the central laboratory in real time and simultaneously for all the acquisition devices. This requires the use and therefore the availability of a large number of wide or narrow band short wave transmission channels of different frequencies.
Transmission of the data collected by the acquisition devices may also be made sequentially, each of them transmitting in turn its own data either directly to the central laboratory or through other intermediate acquisition devices or relay elements. Recording means are then used for storing the collected data for the time required for their sequential transfer to the central
Short wave link seismic data transmission systems are described for example in the U.S. Pat. No. 4 583 206 or French Patent No. 2 538 561.
Delayed transmission systems are also known in particular from the U.S. Pat. No. 3 946 357 and the European patent No. EP 10 660, in which the collected seismic data are stored in cassette recorders contained in the different acquisition apparatus. At the end of each recording session in the field or at regular intervals, the field team replaces all the recording cassettes and brings back the recorded cassettes to the central laboratory where their contents are transferred to a high capacity recording apparatus. When a short wave link is available between each acquisition device and the central laboratory, this latter may order in turn reading of the cassettes of all the recorders and the transfer of the data which they contain to the central recording apparatus.
The prior transmission systems mentioned have drawbacks related not only to the method of connection to the central laboratory but also the type of recorder available in each acquisition device. In fact, seismic prospection operations are often conducted in zones where the local regulations relating to the use of transmission channels involves troublesome restrictions. The transmission of seismic data, when it is made by short wave link, requires one or more wide band transmission channels which it is sometimes difficult to obtain.
The use of a recorder in each acquisition device makes it possible to avoid permanent if not total recourse to short wave links in the case of delayed transmission and so facilitates the preparation of a firing run. But in this case, the advantages are lost which are brought by a permanent control.
Since cassette recorders are not always of a very high reliability, the staff of the central laboratory cannot check whether their operation is correct or detect a possible failure.
Tests are sometimes conducted before the beginning of each recording session on a seismic profile, from the central laboratory and by short wave link, to check the correct operation of the recorders. But these tests are often very long because of the number of acquisition apparatus to interrogate, and cannot be carried out systematically. The field staff may then remain unaware of the possible failures of recorders and more generally of the acquisition apparatus, and that sometimes leads to an important loss of seismic data.
Since cassette recorders are relatively slow, the transfer of seismic data to the central recorder often takes a considerable time. This is true if the transfer takes place by short wave link from each of the acquisition devices. That is also the case when the storage cassettes, once recorded, are brought back to the central laboratory and read one after the other.